The invention relates to a steam foam drive process for producing oil from, or displacing oil within, a subterranean reservoir. The invention is particularly suited for use in reservoirs in which stratification due to differences in the absolute permeabilities of individual layers of rocks does not confine a significant portion of injected steam within the layers of highest absolute permeability. The invention also relates to steam-foam-forming compositions which are uniquely effective in a steam foam drive oil displacement process.
In certain respects, this invention is an improvement in the steam-channel-expanding steam foam drive process of U.S. Pat. No. 4,086,964. The disclosures in and references pertinent to the U.S. Pat. No. 4,086,964 (hereinafter referred to as the '964 patent) are incorporated herein by cross-reference.
The present invention is particularly useful in an oil producing process of the type described in the '964 patent. In the '964 process steam is injected into, and fluid is produced from, horizontally spaced locations within a portion of an oil reservoir in which the disposition of a steam flow path is determined by gravity and/or oil distribution, rather than being substantially confined within layers of relatively high absolute permeability. After a steam channel has been formed and extended until a steam breakthrough into a production location is at least substantially imminent, the composition of the fluid being injected is changed from steam to a steam-foam-forming mixture. The composition of the mixture is correlated with the properties of the rocks and the fluids in the reservoir so that the pressure required to inject the mixture and move it through the steam channel exceeds that required for steam alone but is less than the reservoir fracturing pressure. The composition and rate of injecting the mixture is subsequently adjusted to the extent required to maintain a flow of steam foam within the channel at a relatively high pressure gradient at which the oil-displacing and channel-expanding effects are significantly greater than those provided by the steam alone--without plugging the channel. Oil is recovered from the fluid produced from the reservoir.
As used herein the following terms have the following meanings: "Steam Foam" refers to a foam and/or gas-liquid dispersion which (a) is capable of both reducing the effective mobility, or ease with which a fluid containing such a foam or dispersion will flow within a permeable porous medium and (b) has a gas-phase which is mostly dry (or dry saturated) steam. "Mobility" or "permeability" refers to an effective mobility or ease of flow of fluid within a permeable porous medium. A "permeability reduction" or "mobility reduction" refers to reducing the ease of such a fluid flow due to an increase in the effective viscosity of the fluid and/or a decrease in the effective permeability of the porous medium. A reduction in such a mobility or permeability can be detected and/or determined by measuring differences in internal pressures within a column of permeable porous material during a steady state flow of fluid through a column of such material. "Steam quality" as used regarding any steam-containing fluid refers to the weight percent of the water in that fluid which is in the vapor phase of the fluid at the boiling temperature of that water at the pressure of the fluid. For example: in a monocomponent steam-containing fluid which consists entirely of water and has a steam quality of 50%, one-half of the weight of the water is in the vapor phase; and, in a multicomponent steam-containing fluid which contains nitrogen in the vapor phase and dissolved or dispersed surfactant and electrolyte in the liquid phase and has a steam quality of 50%, one-half the weight of the weight of the water in the multi-component steam-containing fluid is in the vapor phase. Thus, the steam quality of a steam-containing fluid can be calculated as, for example, 100 times the mass (or mass flow rate) of the water vapor in that fluid divided by the sum of the mass (or mass flow rate) of both the water vapor and the liquid water in that fluid. "Steam-foam-forming mixture" (or composition) refers to a steam-containing mixture of steam and aqueous liquid solution or dispersion of surfactant, with some, all, or none, of the steam being present in the gas phase of a steam foam.